Adam,
That's the air recirculation valve on the turbo. Leave it disconnected and plugged, and also disconnect and plug the EGR valve. Both are evil, reduce HP, and reduce MPG. If you want the car to appear stock you can insert a "BB" into the hose at each item, which makes the fix invisible. I plugged mine & removed all of the vacuum hoses, so my engine compartment is much neater.
Regards,